EP3683327B1

EP3683327B1 - Method of manufacturing an almgsc-series alloy product

Info

Publication number: EP3683327B1
Application number: EP19152342.2A
Authority: EP
Inventors: Philipp Daniel RUMPF; Sabine Maria Spangel; Achim BÜRGER
Original assignee: Aleris Rolled Products Germany GmbH
Current assignee: Novelis Koblenz GmbH
Priority date: 2019-01-17
Filing date: 2019-01-17
Publication date: 2021-05-05
Anticipated expiration: 2039-01-17
Also published as: JP7229370B2; CN113302329B; BR112021009928A2; CA3121117A1; JP2022520326A; US20220098715A1; KR20210084532A; EP3683327A1; ES2878315T3; PT3683327T; WO2020148203A1; KR102565389B1; CN113302329A; CA3121117C

Description

FIELD OF THE INVENTION

The invention relates to a method of manufacturing an AIMgSc-series aluminium alloy product. The resultant product provides an improved corrosion resistance. The aluminium alloy product can be in the form of a rolled product (sheet or plate), an extruded product, a forged product, or a powder-metallurgy product.

BACKGROUND OF THE INVENTION

AlMg-series aluminium alloys which may optionally or mandatorily have Sc as alloying element are known in the art, for example from the following documents:
US 6,695,935 B1 (Corus/Aleris) discloses an alloy in the form of a rolled or extruded product and having the composition of: 3.5-6.0% Mg, 0.4-1.2% Mn, 0.4-1.5% Zn, up to 0.25% Zr, up to 0.3% Cr, up to 0.2% Ti, up to 0.5% Fe, up to 0.5% Si, up to 0.4% Cu, one or more selected from the group of (0.005-0.1% Bi, 0.005-0.1% Pb, 0.01-0.1% Sn, 0.01-0.5% Ag, 0.01-0.5% Sc, 0.01-0.5% Li, 0.01-0.3% V, 0.01 -0.3% Ce, 0.01 -0.3% Y, 0.01 -0.3% Ni), others each 0.05%, total 0.15%, balance aluminium. The alloy is said to offer improved long-term corrosion resistance in both soft temper (O-temper) and work- or strain-hardened temper (H-temper) as compared to those of the standard AA5454 alloy.
EP 1 917 373 B1 (Aleris) discloses an aluminium alloy product having 3.5-6.0% Mg, 0.4-1.2% Mn, up to 0.5% Fe, up to 0.5% Si, up to 0.15% Cu, 0.05-0.25% Zr, 0.03-0.15% Cr, 0.03-0.2% Ti, 0.1-0.3% Sc, up to 1.7% Zn, up to 0.4% Ag, up to 0.4% Li, optionally one or more dispersoid-forming elements selected from the group consisting of (Er, Y, Hf, V) each up to 0.5%, impurities or incidental elements each <0.05%, total <0.15%, and the balance aluminium.
RU 2280705 C1 discloses an alloy containing 4.2-6.5% Mg, 0.5-1.2% Mn, up to 0.2% Zn, up to 0.2% Cr, up to 0.15% Ti, up to 0.25% Si, up to 0.30% Fe, up to 0.1% Cu, 0.05-0.3% Zr, at least one element selected from the group consisting of (0.05-0.3% Sc, 0.0001-0.01% Be, 0.001-0.1% Y, 0.001-0.1% Nd, 0.001-0.1% Ce), balance aluminium. The aluminium alloy is said to have improved ballistic properties. The Zn and Si content are reduced to improve the weldability and to increase the corrosion resistance of the aluminium alloy.
RU 2268319 C1 discloses an alloy containing 5.5-6.5% Mg, 0.10-0.20% Sc, 0.5-1.0% Mn, 0.10-0.25% Cr, 0.05-0.20% Zr, 0.02-0.15% Ti, 0.1-1.0% Zn, 0.003-0.015% B, 0.0002-0.005% Be, balance aluminium, and wherein the sum of Sc+Mn+Cr is at least 0.85%.
WO 01/12869 A (Kaiser Aluminum) discloses an alloy comprising 4.0-8.0% Mg, 0.05-0.6% Sc, 0.1-0.8% Mn, 0.5-2.0% Cu or Zn, 0.05-0.20% Hf or Zr, and the balance aluminium and incidental impurities.
WO 98/35068 (Alcoa) discloses an aluminium alloy product comprising 3-7% Mg, 0.03-0.2% Zr, 0.2-1.2% Mn, up to 0.15% Si, and 0.05-0.5% of a dispersoid-forming element selected from the group consisting of (Sc, Er, Y, Ga, Ho, Hf), the balance being aluminium and incidental elements and impurities, and wherein the aluminium alloy product is preferably Zn-free and Li-free.
WO 2018/073533 A1 (Constellium) discloses a method for producing a hot-worked product, in particular a sheet product having a thickness of less than 12 mm, made of an aluminium alloy composed, of Mg: 3.8-4.2%, Mn: 0.3-0.8%, Sc: 0.1-0.3%, Zn: 0.1-0.4%, Ti: 0.01-0.05%, Zr: 0.07-0.15%, Cr: <0.01%, Fe: <0.15%, Si <0.1%, wherein the homogenisation is carried out at a temperature of between 370°C and 450°C, for between 2 and 50 hours, such that the equivalent time at 400°C is between 5 and 100 hours, and the hot deformation is carried out at an initial temperature of between 350°C and 450°C. The products are said to be advantageous as they offer a better compromise in terms of mechanical strength, toughness and hot-formability.

DESCRIPTION OF THE INVENTION

As will be appreciated herein below, except as otherwise indicated, aluminium alloy and temper designations refer to the Aluminium Association designations in Aluminum Standards and Data and the Registration Records, as published by the Aluminium Association in 2018 and are well known to the persons skilled in the art.
For any description of alloy compositions or preferred alloy compositions, all references to percentages are by weight-percent unless otherwise indicated.
The term "up to" and "up to about", as employed herein, explicitly includes, but is not limited to, the possibility of zero weight-percent of the particular alloying component to which it refers. For example, up to 0.10% Zn may include an alloy having no Zn.
It is an object of the invention to provide a method of manufacturing an AlMgSc-series aluminium alloy product having an improved corrosion performance.
It is an object of the invention to provide a method of manufacturing an AIMgSc-series aluminium alloy product having an improved exfoliation corrosion resistance in combination with an improved intergranular corrosion resistance.
These and other objects and further advantages are met or exceeded by the present invention providing a method of manufacturing an AIMgSc-series aluminium alloy product, the method comprising the step of cooling said AIMgSc-series aluminium alloy product from a final annealing temperature or a set annealing temperature to below 150°C, wherein the cooling down in a first temperature range of about 250°C to about 200°C is at an equivalent time of more than 4 hours, preferably more than 6.5 hours and more preferably more than 26 hours, and wherein the cooling down in a second temperature range from about 200°C to about 150°C is at an equivalent time of more than 0.2 hours, preferably more than 0.4 hours and more preferably more than 0.8 hours, and wherein the equivalent time (t(_eq)) is defined as $t (eq) = \frac{\int \exp (- 16000 / T) dt}{\exp (- 16000 / T_{ref})}$
wherein T (in degrees Kelvin) indicates the temperature of the heat treatment, which changes over the time t (in hours) and T_ref (in degrees Kelvin) is the reference temperature selected at 473K (200°C).
The method according to the invention provides AIMgSc-series aluminium alloy products that have a good strength, preferably Rp> 200 MPa, in combination with a good corrosion resistance, in particular a good exfoliation corrosion resistance in combination with a good intergranular corrosion resistance. The cooling rates applied are economically feasible in an industrial environment of manufacturing the AIMgSc-series aluminium alloy products.
The AIMgSc-series aluminium alloy product manufactured in accordance with the invention is resistant to exfoliation corrosion. "Resistant to exfoliation corrosion" means that the aluminium alloy product passes ASTM Standard G66-99 (2013), entitled "Standard Test Method for Visual Assessment of Exfoliation Corrosion Susceptibility of 5XXX Series Aluminium Alloys (ASSET Test)". N, PA, PB, PC and PD indicate the results of the ASSET test, N representing the best result. The aluminium alloy products manufactured in accordance with the invention achieve before and after being sensitised a PB result or better.
The AIMgSc-series aluminium alloy product manufactured in accordance with the invention is also resistant to intergranular corrosion. "Resistant to intergranular corrosion" means that both before and after the AIMgSc-series aluminium alloy has been sensitized, the aluminium alloy product passes ASTM Standard G67-13, entitled "Standard Test Method for Determining the Susceptibility to Intergranular Corrosion of 5XXX Series Aluminium Alloys by Mass Loss After Exposure to Nitric Acid" (NAMLT Test)". If the measured mass loss per ASTM G67-13 is not greater than 15 mg/cm², the sample is then considered not susceptible to intergranular corrosion. If the mass loss is more than 25 mg/cm², the sample is then considered susceptible to intergranular corrosion. If the measured mass loss is between 15 mg/cm² and 25 mg/cm², further checks are then conducted by microscopy to determine the type and depth of attack, whereupon one skilled in the art may determine whether there is intergranular corrosion via the microscopy results. The AIMgSc-series aluminium alloy products manufactured in accordance with the invention achieve a measured mass loss per ASTM G67-13 not greater than 15 mg/cm² both before and after being sensitized. Preferably, the measured mass loss is not greater than 12 mg/cm² and more preferably not greater than 9 mg/cm². "Sensitized" means that the AIMgSc aluminium alloy product has been annealed to a condition representative of at least 20 years of service life. For example, the aluminium alloy product may be continuously exposed to elevated temperature for several days (e.g., a temperature in the range of 100°C to 120°C for a period of about 7 days/168 hours).
The AIMgSc-series aluminium alloy product may realize resistance to stress corrosion cracking and intergranular corrosion as a result of, at least in part, the absence of a continuous film of β-phase particles at the grain boundaries. Aluminium alloy products are polycrystalline. A "grain" is a crystal of the polycrystalline structure of the aluminium alloy, and "grain boundaries" are the boundaries that connect the grains of the polycrystalline structure of the aluminium alloy, "β-phase" is Al₃Mg₂, and "a continuous film of β-phase" means that a continuous volume of β-phase particles is present at the majority of the grain boundaries. The continuity of the β-phase may be determined, for example, via microscopy at a suitable resolution (for example, at a magnification of at least 200X).
In accordance with the invention, it has been found that a very fast cooling rate, for example, by means of quenching from the final annealing temperature to below 150°C, has an adverse effect on the corrosion resistance of the AIMgSc-series aluminium alloy product, in particular on the corrosion resistance tested according to the NAMLT-test after being sensitized. A slower cooling rate results in an enhanced intergranular corrosion resistance.
For the cooling down from the final annealing temperature to about 150°C, particularly in the first temperature range of about 250°C to about 200°C, the equivalent time should be longer than 4 hours, preferably longer than 6.5 hours, more preferably longer than 26 hours, and in the second temperature range of about 200°C to about 150°C, the equivalent time should be longer than 0.2 hours, preferably longer than 0.4 hours, more preferably longer than 0.8 hours. The relatively slow cooling rate is important for the precipitation of discontinuous β-phase particles at the grain boundaries and to avoid the precipitation of a continuous film of β-phase particles, both after cooling to ambient temperature and after the Al-Mg-Sc alloy has been sensitized. The cooling down is preferably performed in a continuous mode such that the metal temperature is continuously reduced over time.
The cooling down from the final annealing temperature to the first temperature range starting at about 250°C is not critical. When employing the method according to the invention on an industrial scale, it can be useful or convenient to apply about the same cooling rate as for the first temperature range.
The further cooling down from about 150°C to below about 85°C is less critical and can be done at a higher cooling rate to minimize the coarsening of precipitates. The cooling rate for the cooling down from about 85°C to ambient temperature is not critical.
In an embodiment, the AIMgSc-series aluminium alloy product is in a form selected from the group consisting of a rolled product (sheet or plate), an extruded product, a forged product, and a powder-metallurgy product. In a further embodiment, any of these products are in a welded condition or in a formed condition.
In a particular embodiment, the AIMgSc-series aluminium alloy product is in the form of a rolled product. In a further embodiment, the rolled product has been welded or formed.
In an embodiment, the thickness of the AIMgSc-series aluminium alloy rolled product is at most 25.4 mm (1 inches), and preferably at most 12 mm (0.47 inches), and more preferably 6 mm (0.24 inches), and most preferably 4.5 mm (0.18 inches). In an embodiment, the thickness of the AIMgSc-series aluminium rolled product is at least 1.2 mm (0.047 inches).
In a particular embodiment, the AIMgSc-series aluminium alloy product is in the form of an extruded product.
In an embodiment, the AIMgSc-series aluminium alloy rolled product is cast, subsequently rolled to final gauge, and annealed. The alloy can be provided as an ingot or slab for fabrication into rolling feedstock using casting techniques regular in the art for cast products, e.g. Direct Chill DC-casting, and preferably having an ingot thickness in a range of about 220 mm or more, e.g., 400 mm, 500 mm, or 600 mm. In another embodiment, thin gauge slabs resulting from continuous casting, e.g. belt casters or roll casters, also may be used having a thickness of up to about 40 mm. After casting the rolling feedstock, the thick as-cast ingot is commonly scalped to remove segregation zones near the cast surface of the ingot.
Preferably, the rolling process applied comprises hot rolling and optionally comprises hot rolling followed by cold rolling to final gauge, and where applicable, an intermediate annealing is applied either before the cold rolling operation or during the cold rolling operation at an intermediate cold rolling gauge.
Prior to hot rolling, the AIMgSc-series aluminium alloy product is homogenised or pre-heated for up to about 50 hours, preferably up to about 24 hours, at a temperature in a range of about 320°C to 470°C, preferably of about 320°C to 450°C.
In an embodiment following the hot rolling operation, the hot rolled product receives a very mild cold rolling step (skin rolling or skin pass) with a reduction of less than about 1%, preferably less than about 0.5%, to improve the flatness of the rolled product. In an alternative embodiment, the hot rolled product can be stretched. This stretching step can be carried out with a reduction of up to 3%, preferably between about 0.5% to 1%, to improve the flatness of the hot rolled product.
The final annealing or annealing heat-treatment at final gauge is to recover the microstructure and is typically performed at a set annealing temperature in the range of 250°C to 400°C, preferably in the range of 260°C to 375°C, and more preferably in a range of about 280°C to 350°C, for a time in a range of about 0.5 hours to 20 hours, and preferably of about 0.5 hours to 10 hours.
In an embodiment, the AIMgSc-series aluminium alloy extruded product is produced by a method that comprises the steps, in that order, of: (a) providing an extrusion ingot, e.g. by means of DC-casting, of the aluminium alloy as herein described and claimed; (b) preheating and/or homogenisation of the extrusion ingot; preferably at temperature and times similar as for the rolling feedstock; (c) hot extruding the ingot into an extruded profile having a section or wall thickness in a range of 1 mm to about 20 mm, preferably 1 mm to about 15 mm; the billet temperature at the start of the extrusion process is typically in a range of about 400°C to about 500°C; optionally stretching of the extruded profile to increase product straightness, and (d) annealing of the extruded profile at a final annealing temperature followed by the cooling procedure in accordance with the present invention.
In an embodiment of the invention, the method of cooling the aluminium alloy product is applied immediately following a high-temperature forming operation for shaping the AIMgSc-series aluminium alloy product into a single- or double-curved shape product. The high-temperature forming operation is performed at the final annealing temperature in the range of 180°C to 500°C, preferably in the range of 250°C to 400°C, more preferably in a range of 260°C to 375°C, and most preferably in a range of 280°C to 350°C, for example at about 300°C or at about 325°C. A particular preferred embodiment of such a high-temperature forming operation at the final annealing temperature is by means of a creep forming operation or a relaxation forming operation. Creep forming is a process or operation of restraining a component to a specific shape during heat treatment, allowing the component to relieve stresses and creep to contour, for example, fuselage shells with a double curvature. This creep forming process is, for example, explained in the paper by S. Jambu et al., "Creep forming of AIMgSc alloys for aeronautic and space applications", published at the occasion of the ICAS-2002 congress.
In a preferred embodiment of the high-temperature forming operation at the final annealing temperature into a single- or double-curved shape product, a rolled AIMgSc-series aluminium alloy product is being employed. The AIMgSc-series aluminium alloy product can be provided in an annealed condition also manufactured by the method according to this invention.
Optionally, also extruded AIMgSc-series aluminium alloy products are being employed, for example, as extruded stringers as part of a fuselage panel.
In an embodiment of the invention, the method of cooling the aluminium alloy product is applied on a welded product or panel incorporating the AIMgSc-series aluminium alloy product immediately following a post-weld heat-treatment to recover some strength in particular by reprecipitating AlScZr dispersoids. The post-weld heat-treatment is performed at a temperature similar as for the final anneal heat-treatment and is in the range of 250°C to 400°C, preferably in the range of 260°C to 375°C, and more preferably in a range of about 260°C to 350°C, for a time in a range of about 0.5 hours to 20 hours, and preferably of about 0.5 hours to 10 hours.
In an embodiment of the invention, the method of cooling the aluminium alloy product is applied on a cold-formed and shaped product from the AIMgSc-series aluminium alloy whereby an annealing heat-treatment is performed to reduce residual stress in the cold-formed and shaped product or to recover certain engineering properties such as elongation or damage tolerance. Such an annealing heat-treatment is performed at a temperature similar as for the final anneal heat-treatment and is in the range of 250°C to 400°C, preferably in the range of 260°C to 375°C, and more preferably in a range of about 280°C to 350°C, for a time in a range of about 0.5 hours to 20 hours, and preferably of about 0.5 hours to 10 hours.
In an embodiment, the AIMgSc-series aluminium alloy has a composition comprising, in wt.%:

Mg 3.0% to 6.0%, preferably 3.2%-4.8%, more preferably 3.5% to 4.5%,
Sc 0.02% to 0.5%, preferably 0.02%-0.40%, more preferably 0.05%-0.3%,
Mn up to 1%, preferably 0.3% to 1.0%, more preferably 0.3% to 0.8%,
Zr up to 0.3%, preferably 0.05% to 0.3%, more preferably 0.07% to 0.15%,
Cr up to 0.3%, preferably 0.02% to 0.2%,
Ti up to 0.2%, preferably 0.01 % to 0.2%,
Cu up to 0.2%, preferably up to 0.1 %, more preferably up to 0.05%,
Zn up to 1.5%, preferably up to 0.8%, more preferably 0.1 % to 0.8%,
Fe up to 0.4%, preferably up to 0.3%, more preferably up to 0.20%,
Si up to 0.3%, preferably up to 0.2%, more preferably up to 0.1 %,
impurities and balance aluminium. Typically, such impurities are present each <0.05% and total <0.15%.

The Mg is the main alloying element in the AIMgSc-series alloys, and for the method according to this invention, it should be in a range of 3.0% to 6.0%. A preferred lower-limit for the Mg-content is about 3.2%, more preferably about 3.8%. A preferred upper-limit for the Mg-content is about 4.8%. In an embodiment the upper-limit for the Mg-content is about 4.5%.
Sc is another important alloying element and should be present in a range of 0.02% to 0.5%. A preferred lower-limit for the Sc-content is about 0.05%, and more preferably about 0.1%. In an embodiment, the Sc-content is up to about 0.4%, and preferably up to about 0.3%.
Mn may be added to the AIMgSc-series aluminium alloys and may be present in a range of up to about 1%. In an embodiment, the Mn-content is in a range of about 0.3% to 1%, and preferably about 0.3% to 0.8%.
To make Sc more effective, it is preferred to also add Zr in a range of up to about 0.3%. In an embodiment, the Zr is present in a range of 0.05% to 0.30%, preferably in a range of about 0.05% to 0.25%, and more preferably is present in a range of about 0.07% to 0.15%.
Cr can be present in a range of up to about 0.3%. When purposively added, it is preferably in a range of about 0.02% to 0.3%, and more preferably in a range of about 0.05% to 0.15%. In an embodiment, there is no purposive addition of Cr and it can be present up to 0.05%, and preferably is kept below 0.02%.
Ti may be added up to about 0.2% to the AIMgSc alloy as strengthening element or for improving the corrosion resistance or for grain refiner purposes. A preferred addition of Ti is in a range of about 0.01% to 0.2%, and preferably in a range of about 0.01% to 0.10%.
In an embodiment, there is a purposive combined addition of Zr+Cr+Ti. In this embodiment, the combined addition is at least 0.15% to achieve sufficient strength, and preferably does not exceed 0.30% to avoid the formation of too large precipitates.
In another embodiment, there is a purposive combined addition of Zr and Ti but no purposive addition of Cr. In this embodiment, the combined addition of Zr+Ti is at least 0.08%, and preferably does not exceed 0.25%, and wherein Cr is up to 0.02%, and preferably only up to 0.01%.
Zinc (Zn) in a range of up to 1.5% can be purposively added to further enhance the strength in the aluminium alloy product. A preferred lower limit for the purposive Zn addition would be 0.1%. A preferred upper limit would be about 0.8%, and more preferably 0.5%, to provide a balance in strength and corrosion resistance.
In an embodiment, the Zn is tolerable impurity element, and it can be present up to 0.15%, and preferably up to 0.10%.
Cu can be present in the AIMgSc-alloy as strengthening element in a range up to about 2%. However, in applications of the product where the corrosion resistance is a very critical engineering property, it is preferred to maintain the Cu at a low level of about 0.2% or less, and preferably at a level of about 0.1% or less, and more preferably at a level of 0.05% or less. In an embodiment the Cu-content is 0.01% or less.
Fe is a regular impurity in aluminium alloys and can be tolerated up to about 0.4%. Preferably it is kept to a level of up to about 0.3%, and more preferably up to about 0.20%.
Si is also a regular impurity in aluminium alloys and can be tolerated up to about 0.3%. Preferably it is kept to a level of up to 0.2%, and more preferably up to 0.10%.
In an embodiment, the AIMgSc-series aluminium alloy has a composition consisting of, in wt.%: Mg 3.0% to 6.0%, Sc 0.02% to 0.5%, Mn up to 1%, Zr up to 0.3%, Cr up to 0.3%, Ti up to 0.2%, Cu up to 0.2%, Zn up to 1.5%, Fe up to 0.4%, Si up to 0.3%, balance aluminium and impurities each <0.05% and total <0.15%, and with preferred narrower compositional ranges as herein described and claimed.
In accordance with the invention, it has been found that the method can be employed to a wide range of AIMgSc-series aluminium alloys. It has been found that with increasing Cu-content in the aluminium alloy, a lower cooling rate and thus a longer equivalent time in the defined first and second temperature range from the final annealing temperature is being preferred. Such a very low cooling rate has no adverse effect on the corrosion performance of AIMgSc-series aluminium alloys having a very low Cu-content, for example, less than 0.05%, or even less than 0.01%.
In an embodiment, the aluminium alloy product is a single or double curved panel, in particular, a single or double curved aircraft fuselage panel.
The invention will now be illustrated with reference to the following non-limiting example, both according to the invention and comparative.

EXAMPLE

Sheet products of 4.5 mm have been manufactured on an industrial scale comprising the steps of DC-casting of a rolling ingot, scalping, milling, preheating to hot rolling temperature between 400°C and 450°C, hot rolling, cold rolling to 4.5 mm and with intermediate annealing during the cold rolling operation, and final annealing at a set temperature of 325°C (598K) for 2 hours and followed by different controlled cooling rates according to Table 1 and whereby specimen A, B and C are according to the invention, and specimen D is comparative.
The AIMgSc aluminium alloy cast has the following composition, in wt.%, 4.0% Mg, 0.55% Mn, 0.2% Sc, 0.3% Zn, 0.1% Zr, 0.07% Cr, 0.07% Ti, 0.02% Si, 0.02% Fe, 0.006% Cu, balance aluminium and inevitable impurities.

Table 1 lists the measured mass loss per ASTM G67-13 for each specimen having different cooling regimes from the final annealing temperature after sensitising at 120°C for 168 hours.

Table 1

specimen	cooling after final annealing at 598K				mass loss [mg/cm²]
	between 523K-473K		between 473K-423K
	cooling rate [K/h]	equivalent time t(eq) [hrs]	cooling rate [K/h]	equivalent time t(eq) [hrs]
A	95	4.15	60	0.22	8.7
B	60	6.58	30	0.43	7.4
C	15	26.31	15	0.87	6.0
D	60	6.58	air cooling in still air at RT		16.5

The AIMgSc-series aluminium alloy rolled product manufactured in accordance with the invention is resistant to intergranular corrosion. "Resistant to intergranular corrosion" means that both before and after the AIMgSc-series aluminium alloy has been sensitized, the aluminium alloy product passes ASTM Standard G67-13, (NAMLT Test)". All sensitized specimesn had a PA performance, and all non-sensitized specimen had also a PA performance.
From the results of Table 1, it can be seen that the AIMgSc-series aluminium alloy rolled products manufactured in accordance with the invention achieve a measured mass loss per ASTM G67-13 not greater than 15 mg/cm² after being sensitized. The better examples have a mass loss not greater than 9 mg/cm². With a slower cooling rate or a longer equivalent time in the defined temperature range, the mass loss is further reduced. Specimen D had a too fast cooling rate in the temperature range of 473K to 423K and was thus outside the invention, resulting in a significantly increased mass loss per ASTM G67-13.
Thus, the method according to the invention results in an aluminium alloy product having a good intergranular corrosion resistance in combination with a good exfoliation corrosion resistance.
Similar corrosion performance of the aluminium alloy product will be achieved in the cooling down from a high-temperature forming operation performed at the final annealing temperature, for example, a creep forming operation performed at 310°C or 325°C.
The invention is not limited to the embodiments described before, and which may be varied widely within the scope of the invention as defined by the appending claims.

Claims

A method of manufacturing an AIMgSc-series aluminium alloy product, the method comprising the step of cooling said AIMgSc-series aluminium alloy product from a final annealing temperature to below 150°C, wherein the cooling in a first temperature range of 250°C to 200°C is at an equivalent time of more than 4 hours, and wherein the cooling in a second temperature range from 200°C to 150°C is at an equivalent time of more than 0.2 hours, and wherein the equivalent time (t(_eq)) is defined as $t (eq) = \frac{\int \exp (- 16000 / T) dt}{\exp (- 16000 / T_{ref})}$
wherein T (in degrees Kelvin) indicates the temperature of the heat treatment, which changes over the time t (in hours) and T_ref (in degrees Kelvin) is the reference temperature selected at 473K (200°C).
Method according to claim 1, wherein the equivalent time in the first temperature range is longer than 6.5 hours, and preferably longer than 26 hours.
Method according to claim 1 or 2, wherein the equivalent time in the second temperature range is longer than 0.4 hours, and preferably longer than 0.8 hours.
Method according to any one of claims 1 to 3, wherein the final annealing temperature is in a range of 250°C to 400°C, preferably in a range of 260°C to 375°C, and more preferably in a range of 280°C to 350°C.
Method according to any one of claims 1 to 4, wherein said AIMgSc-series aluminium alloy product is in a form selected from the group consisting of a rolled product, an extruded product, a forged product, a powder-metallurgy product.
Method according to any one of claims 1 to 5, wherein said AIMgSc-series aluminium alloy product is a rolled product.
Method according to claim 1 to 6, wherein the rolled product has a thickness of up to 25.4 mm, and preferably of up to 12 mm, more preferably up to 6 mm.
Method according to any one of claims 1 or 7, wherein the method comprises the steps of casting an AIMgSc-series aluminium alloy ingot, rolling the ingot to final gauge into a rolled product, and heat-treating by annealing of the rolled product at the final annealing temperature, followed by cooling in accordance with any one of claims 1 to 3.
Method according to any one of claims 1 to 8, wherein the method comprises the steps of a high temperature forming operation of an AIMgSc-series aluminium alloy product into a single- or double-curved shape product at the final annealing temperature followed by cooling in accordance with any one of claims 1 to 3.
Method according to claim 9, wherein the high temperature forming operation at the final annealing temperature is by a creep forming operation or a relaxation forming operation.
Method according to any one of claims 1 to 10, wherein the AIMgSc-series aluminium alloy product has a composition comprising of, in wt.%: Mg 3.0% to 6.0%, preferably 3.2% to 4.8%, Sc 0.02% to 0.5%, preferably 0.02% to 0.40%, Mn up to 1%, Zr up to 0.3%, preferably 0.05% to 0.3%,

impurities and balance aluminium.
Method according to any one of claims 1 to 11, wherein the AIMgSc-series aluminium alloy product has a composition comprising of, in wt.%: Mg 3.0% to 6.0%, preferably 3.2% to 4.8%, Sc 0.02% to 0.5%, preferably 0.02% to 0.40%, Mn up to 1%, preferably 0.3% to 1.0%, Zr up to 0.3%, preferably 0.05% to 0.3%, Cr up to 0.3%, Ti up to 0.2%, preferably 0.01 % to 0.2%, Cu up to 0.2%, Zn up to 1.5%, Fe up to 0.4%, Si up to 0.3%,

impurities and balance aluminium.
Method according to any one of claims 1 to 12, wherein the AlMgSc-series aluminium alloy product manufactured in accordance with the invention achieves a measured mass loss per ASTM G67-13 not greater than 15 mg/cm², both before and after being sensitized, and preferably the mass loss is not greater than 12 mg/cm², and more preferably not greater than 9 mg/cm².